In container yards, mobile gantry cranes are used for lifting, moving and stacking freight containers.
A first type of such a gantry crane moves with steel wheels on a rail track, which consists of two rails bordering a substantially rectangular container stacking area lengthwise. These first gantry cranes are referred to as rail-mounted gantry cranes (RMG cranes). RMG cranes are generally electrically powered via a power cable wound on a cable reel or winch mounted on one side of the gantry crane. To avoid damaging the power cable, the latter is—during movement of the RMG along its rail track—placed in a trench extending along one of the rails of the rail track, plumb-vertically under the horizontal trajectory of the cable reel or winch. A major disadvantage of RMG cranes is that for servicing two container stacking areas not located along the same rail track, two different RMG cranes are required, because the RMG crane cannot travel from a first rail track onto a second rail track.
Another type of gantry crane used in container yards has wheels with rubber tires instead of steel wheels and is therefore generally referred to as a rubber tired gantry crane (RTG crane). Such RTG crane is not limited to moving along a specific rail track. With its rubber tires, the RTG crane moves on prepared surfaces known as runways, wherein two runways bordering a container stacking form a so called lane. RTG crane wheels are generally steerable for changing the direction of travel of the RTG crane, for example for moving from one container stacking area to another, i.e. from a first lane to a second lane. Consequently, RTG cranes provide a greater flexibility of use than RMG cranes. However, as an RTG crane must be capable of moving between two distant container stacking areas, they can no longer be powered by a fixed connection to electric power mains.
Most RTG cranes are therefore powered by an on-board diesel engine coupled to an electric generator. On the RTG crane, the generator powered by the diesel engine supplies electric power to electric motors for moving the RTG crane and operating the hoist and other equipment. However, environmental and maintenance issues, as well as strongly increasing diesel prices are making onboard diesel engines less and less attractive.
Consequently, more and more RTG cranes use their diesel engine only for moving from one container stacking area to another, i.e. for so-called cross-lane manoeuvres. When operating along a straight lane for servicing one specific container stacking area, the diesel engine of such an RTG crane is shut down and its electric motors are powered with electricity from the electric power mains.
A first system for supplying electricity from the electric power mains to an RTG crane travelling along a lane comprises a conductor rail systems extending along a runway of the lane. On the RTG crane is mounted a self engaging current collector trolley. When the RTG crane arrives at the new lane, its collector trolley automatically engages the conductor rails, so that the RTG crane is automatically connected to the electric power mains. It will however be noted that the collector trolley and/or the conductor rails are easily damaged, and that the collector rails present moreover a high risk of electrocution.
According to an alternative system, electric power is supplied to the RTG crane through a power cable, just as explained above for an RMG crane. This means that as the RTG crane moves along the RTG lane, the power cable is lifted from a cable trench or a cable path extending along a runway of the lane and wound around a crane-mounted cable reel. When the RTG crane moves in the opposite direction along the lane, the power cable is unwound from this cable reel and placed back into the cable trench or on the cable path.
Such an RTG crane is, for example, disclosed in EP 1 820 769 A1 (the reference numbers used in the present paragraph refer to the reference numbers used in this prior art document). In order to allow cross-lane manoeuvres, a so-called joint-box 20 is associated with each cable trench 22, and the free end of the power cable 18 is equipped with a plug which is removably connectable to a socket in the joint-box 20. Prior to carrying out a cross-lane manoeuvre between a first lane and a second lane, the plug of the power cable 18 is disconnected from the socket in the joint-box 20 of the cable trench 22 extending along the first lane. The RTG crane can now travel with its on-board diesel engine from the first to the second lane and—after it is properly positioned in the second lane—the plug of the power cable is connected to the socket in the joint-box of the cable trench extending along this second lane.
It will be appreciated that manually disconnecting the plug of the power cable from the socket in the joint-box of the first cable trench and manually reconnecting it to the socket of the joint-box of the second cable trench, are time consuming, burdensome and potentially dangerous. They are time consuming, because the crane operator must leave his control cabin, which is normally fixed to a trolley at the top of the gantry crane, must descend to ground-level, must perform the connection or disconnection, and must then return to his control cabin. If the connection/disconnection is to be carried out by another person than the crane operator, that person must be available in time for performing the connection or disconnection; otherwise even more time will be lost. These operations are burdensome tasks, because it is not easy to guide the plug with the heavy cable attached thereto into its socket arranged in a pit at floor level. Furthermore, if the connection/disconnection is made by another person than the crane operator, there is also a danger that the person making the connection/disconnection may be hit by the moving RTG crane.
Chinese utility patent CN 202148142 U discloses a device for automatically connecting a mobile gantry crane via a cable to an electric power supply. This mechanism comprises a plug frame, a socket frame, a connection mechanism and a locking mechanism. The plug frame includes a first connector part connected to a free end of the cable. The socket frame includes a second connector part configured for mating with the first connector part according to a vertical coupling axis, when the plug frame is vertically deposited into the socket frame. The connection mechanism is arranged on the gantry crane and comprises a horizontal expansion module and an up-down moving module. It supports the plug frame via detachable coupling mechanism and deposits it into the socket frame, wherein a funnel-shaped guide means laterally aligns the plug frame with the socket frame. Before the connection mechanism is decoupled from the plug frame, the latter is locked in the socket frame by means of a locking mechanism, comprising an actuator driving e.g. a locking pin. Such a locking mechanism has however many drawbacks. For example, if the locking mechanism does not properly unlock during the disconnecting procedure, the whole system may be seriously damaged. Similarly, if the locking mechanism does not properly lock the plug frame in the socket frame during the connecting procedure, the plug frame will be ripped out of the socket frame by the gantry crane and, if nothing else, the connectors will be destroyed.
Consequently, there is a need for a simple and cost efficient device for connecting a vehicle, in particular an RTG crane, to an electric power supply, which would, in principle, make a manual intervention in the power cable connection/disconnection operations superfluous.
JP 2011-073846 A discloses a crane feeder system for feeding a crane, which handles ocean transport containers, with electric power from a feeder placed along a lane. The system comprises a feeding carriage which is moved along the feeder. A connecting device includes a male portion connected to a free cable end hanging down from the crane and a female portion mounted on the feeding carriage. The male portion has a conical body with a plurality of electrode rings. The female portion has a conical cavity for receiving the conical body of the male portion, and a plurality of electrode rings capable of mating with the electrode rings of the male portion. Connection is achieved by lowering the conical body of the male portion vertically into the conical cavity of the female portion. This Japanese document does not appear to disclose any locking means for locking the male portion within the female portion. It follows that the male portion and the female portion may disconnect when the crane is moved relative to the feeding carriage.